1. The Field of the Invention
The present invention generally relates to bearing assemblies. More particularly, the present invention relates to a simplified bearing assembly design that enhances heat dissipation in apparatus such as x-ray generating devices.
2. The Related Technology
X-ray producing devices are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. For example, such equipment is commonly employed in areas such as medical diagnostic examination and therapeutic radiology, semiconductor fabrication, and materials analysis.
Regardless of the applications in which they are employed, x-ray devices operate in similar fashion. X-rays are produced in such devices when electrons are emitted, accelerated, then impinged upon a material of a particular composition. This process typically takes place within an evacuated enclosure, in which is disposed a cathode for emitting electrons, and an anode assembly, which comprises a bearing assembly, a rotor shaft, and an anode mounted to the rotor shaft and oriented to receive the electrons. The rotor shaft, in turn, is rotatably supported by the bearing assembly.
A typical x-ray tube bearing assembly generally comprises a bearing housing having a cylindrical cavity in which is disposed a shaft. Further, first and second bearing sets are disposed near each end of the bearing housing cavity in such a manner as to permit free rotation of the shaft. Each bearing set comprises a plurality of balls confined between an inner race defined by the shaft, and an bearing ring defined by an annular ring disposed within the bearing housing cavity. Also disposed within the housing cavity is a hollow cylindrical bearing spacer concentrically disposed about the central portion of the shaft and interposed between the two bearing sets to maintain a predetermined distance between them.
To produce x-rays, an electric current is supplied to a filament disposed in the cathode, causing the filament to emit a cloud of electrons by thermionic emission. A high electric potential imposed between the cathode and anode causes electrons in the cloud to accelerate toward a target surface located on the anode. Upon striking the target surface, the electrons are decelerated and thereby convert their kinetic energy into electromagnetic radiation of very high frequency, i.e., x-rays. The specific frequency of the x-rays produced depends in large part on the type of material used to form the anode target surface. Target surface materials with high atomic numbers (“Z numbers”) are typically employed. The x-rays are then collimated so that they exit the x-ray device through a window disposed in the evacuated enclosure, and enter an x-ray subject, such as a medical patient.
While some of the electrons emitted by the cathode produce x-rays, the majority does not however, and instead convert their kinetic energy to heat upon impact with the anode or other x-ray tube components. A significant amount of the heat created by these electrons is conducted through the anode to the rotor shaft and supporting bearing assembly.
The heat produced by such electrons may, if left unchecked, cause severe damage to the x-ray tube. For instance, the bearing sets disposed in the bearing assembly are especially sensitive to heat. Excessively high temperatures produced in the anode and conducted through the rotor shaft and shaft to the bearing sets can melt the thin metal lubricant that surrounds the bearings, thereby causing the lubricant to disperse and exposing the bearings to excessive friction. The lubricant may also form clumps as a result of excessive exposure to heat, which in turn causes the bearing assembly to operate in a noisy and less smooth manner. Additionally, repeated exposure to high temperatures can gradually degrade the integrity of the bearing surfaces, thereby reducing their useful life or even causing premature bearing failure. These and other effects caused by excessive heating of the anode assembly can ultimately shorten the operational life of the x-ray tube. Therefore, it is important to reliably and continuously dissipate heat from the bearing assembly.
Hollow rotor shafts may be of some benefit in limiting the amount of heat that is conducted from the anode to the bearing assembly because they are relatively more resistant to heat conduction than a solid shaft. In some cases, application of emissive coatings to the rotor shaft may enhance its heat radiation capabilities. Such techniques may not be sufficiently effective in all cases however.
Typically, efficient heat removal from the shaft is hindered by at least two conditions. First, in order for the heat to be removed from the shaft and transmitted to the bearing housing, a significant portion of the heat must pass through the bearing spacer disposed concentrically about the shaft. This configuration implicates a lower rate of heat transfer between the shaft and the bearing housing than is desired. Second, the gap that must exist between the inner diameter of the bearing spacer and the outer diameter of the shaft further slows the rate of heat transfer to the bearing housing. As a result of these conditions, efficient heat transfer between the shaft and bearing housing is prevented, and an unacceptable amount of heat is transmitted to the bearing sets. As described above, such a situation may result in severe damage to the bearing assembly.
Various aspects concerning the assembly of the x-ray tube may be problematic as well. For example, the fit between the outer diameters of the bearing spacer and outer bearing races, and the inner diameter of the cavity of the bearing housing, typically must be tight to maximize contact therebetween and thereby facilitate heat transfer. If the fit is too loose, excessive play will be introduced into the bearing sets, thereby increasing wear and reducing their operational lives. If the fit is too tight, however, particles may be created as the bearing spacer and outer bearing races are inserted into the bearing housing cavity. Later, when the x-ray tube is operated, the particles may migrate to and infiltrate the bearing set. Such particles, may impede bearing motion and significantly increase ball bearing friction, thereby reducing the operational longevity of the bearing sets, and increasing the likelihood of premature bearing failure.
In light of the foregoing, a need therefore exists for a bearing assembly that includes features directed to maximizing the rate at which heat can be transferred away from the bearing assembly. Further, the bearing assembly should be easy to assemble and should facilitate reliable operation of the x-ray device.